1. Field of the Invention
The present invention relates to a semiconductor laser controller.
2. Description of the Prior Art
Semiconductor laser controllers have been proposed for use in an optical disk apparatus of an addition record type which records data by directly modulating a semiconductor laser by recording signals and forming holes in a rotating disk-shaped recording medium.
A typical semiconductor laser controller has been disclosed in Japanese Patent Application No. 57-34903 of the applicant of the present application. In this conventional semiconductor controller, the output of a semiconductor laser is monitored continuously with an optical detector accommodated together with the semiconductor laser in a package, the output of the semiconductor laser is sampled directly when pulse modulation is not executed or the bottom output monitor voltage is sampled when pulse modulation is executed, by means of a peak detection circuit or a sample hold circuit, and the output or the bottom output monitor voltage is compared with a reference voltage (V.sub.B) corresponding to a predetermined bottom output for the feedback control of the semiconductor laser driving current.
A semiconductor laser controller of such a construction, however, has a problem that since only the bottom output is controlled when pulse modulation is executed, the peak output changes under the influence of the change of the characteristics of the semiconductor laser with temperature and passage of time. A semiconductor laser is subject to great variation with temperature and passage of time in respect of oscillation threshold value and quantizing efficiency. An object of the above-mentioned semiconductor laser controller is to restrict the output variation of a semiconductor laser resulting from the change of the oscillation threshold value with temperature and passage of time to the least extent. FIG. 1 is a graphical representation of laser output characteristics, for assistance, in explaining the functions of a conventional semiconductor laser controller, in which the relation of semiconductor laser driving current to semiconductor laser output characteristics is shown. In FIG. 1, an arrow G indicates the change of the output characteristics of a semiconductor laser from a curve A to a curve B due to the change of the oscillation threshold. An arrow H indicates the output characteristics of the same from a curve B to a curve C indicated by broken line due to the change of the quantization efficiency. In FIG. 1, V.sub.B is bottom output monitor voltage, V.sub.p is peak output monitor voltage when pulse modulation is executed, V.sub.R is modulated pulse voltage amplitude, and R.sub.L is the load resistance of a semiconductor laser.
Accordingly, modulated pulse current amplitude I.sub.R =V.sub.R /R.sub.L.
The above-mentioned semiconductor laser controller stabilizes the bottom output by shifting the semiconductor laser driving current from a to b, when the output characteristics of the semiconductor laser changes from the curve A to the curve B, and also achieves indirectly the stabilization of the peak output by a fact that the variation of the peak output is limited to the least extent if the modulated pulse voltage amplitude V.sub.R is fixed, provided that the quantization efficiency remains unchanged.
However, if the quantization efficiency changes, the peak output changes accordingly.
If the peak output of the semiconductor laser is controlled through a feedback control system, every start of pulse modulation requires drawing action and the setting time is too long to be neglected as compared with the period of pulse modulation. An attempt to reduce the setting time causes wide variation of the peak output at the start of modulation, which causes problems, such as unsatisfactory recording and reproduction, when the semiconductor laser is applied to an apparatus, such as an optical disk apparatus, and hence the reduction of the setting time has been infeasible.